Fire code, I assume. You put a door on it, now it's a room, a bedroom at that which requires two exits in case of a fire. Hang a curtain, it's just an overbui

Fire code, I assume. You put a door on it, now it's a room, a bedroom at that which requires two exits in case of a fire. Hang a curtain, it's just an overbuilt loft bed, furniture, invisible to the fire inspector.